Process for making expanded polymeric product with low level of emission of blowing agent

ABSTRACT

A method for making low density (0.8-2 lb/cu.ft.) expanded polymeric products uses blowing agent in an amount of only from 2 to 4.4 weight percent, the process emitting blowing agent during processing in an amount of from only about 0.3 to about 2.5 weight percent. The process uses 2 to 5 expansion steps together with a polymer having a particular polydispersity, weight average molecular weight and Mz:Mn, this polymer having a greater expandability than conventional polymers. The process requires only about half of the amount of blowing agent currently being used in commercially viable processes for making expanded polystyrene products. The process can be used with or without a molding step.

This is a division of application Ser. No. 07/618,342 filed Nov. 26,1990, now U.S. Pat. No. 5,086,078.

BACKGROUND OF THE INVENTION

The present invention relates to a process for making a relatively lowdensity expanded polymeric product while emitting only a low level ofblowing agent. The effect of this process is that during the process, asmaller amount of volatile organic compounds (i.e. pollutants) areemitted into the atmosphere.

The process and product cf the present invention are achieved by using:(1) a low initial level of blowing agent, in combination with (2) apolymer which is highly expandable, in order to make a low densityproduct, while (3) emitting only a low level of the blowing agent duringthe process. The final result is a process which produces a low densityproduct while emitting less blowing agent. The reduction in emission ofblowing agent produces not only a cost savings due to the use of lessblowing agent, but more importantly, reduced release ofenvironmentally-damaging volatile organic compounds into the atmosphere.The process of the present invention results in a previously unachievedlow level of emissions of volatile organic compounds during a commercialprocess of manufacturing the foam product.

Methods for the production of molded polystyrene product have requiredmaking an intimate mixture of a polystyrene polymer and a blowing agent.In commercial operations, this intimate mixture was generally made intosolid (relatively "high-density") beads of relatively small size (e.g.beads having a diameter of from about 0.2 to 4 millimeters). The beadswere then expanded (via heating the mixture) in order to make anexpanded polystyrene product. The expansion was usually carried out byheating the beads above their softening temperature and above theboiling point of the blowing agent (usually pentane), resulting in thevaporization of the blowing agent [The blowing agent must have a boilingtemperature below the softening temperature of the polymer blowing agentmixture.] The vaporization of the blowing agent caused an expansion ofthe beads to form individual particles of foam. The expansion wasgenerally carried out by using a first expansion step, whereafterexpanded particles (referred to as prepuff) were then placed into a moldand again heated, whereby the prepuff further expanded and, because ofthe confined volume, fused to form a unitary object. Under optimalconditions, the bonds formed between the individual prepuff particleswere stronger than the individual particles themselves. That is, uponstressing the finished, molded object enough to cause it to break, thebreak would occur mostly across the individual prepuff particles, ratherthan at the junctions and interstices of the prepuff particles.

Fundamental to the foaming operation is the requirement that the polymercontain a blowing agent. In prior commercial production, steam has beenused to heat the beads, the steam causing the blowing agent to vaporize,which vaporization results in the formation of gaseous bubbles withinthe polymer. These bubbles expand as the internal pressure increases. Afoam is produced because the bubbles, for the most part, are trapped,resulting in the production of a foam. Since the vast majority of thevolume within the foam is occupied by these bubbles, the resulting foamhas a density much lower than that of the unexpandable polymer/blowingagent mixture. The blowing agent also diffuses out over time.

A large fraction of the foamed polystyrene currently being produced hasa density of from 0.8 lb./cu.ft. to 1.1 lb./cu.ft. This density ofmaterial is generally used for building insulation and/or for protectivepackaging. In order to achieve this density, it has been the practice ofthe commercial manufacturers of such "low-density" polystyrene foam toincorporate from about 6 to about 8 weight percent of pentane into thepolystyrene polymer. Beads are formed via a suspension polymerizationduring or after which pentane is introduced into the beads. These beadsare then heated in a single premolding expansion step in which the beadsare expanded in volume by a factor of about 40 (i.e. to a density ofabout 1.0 lb/cu.ft.). The now preexpanded beads (prepuff) are allowed tocool and equilibrate by permitting air to diffuse thereinto, and arethen put into a mold where they are again heated, resulting in thefurther expansion and fusion of the preexpanded prepuff, so that theprepuff particles are bonded together.

U.S. Pat. No. 2,884,386 describes a process for making cellular bodiesof organic thermoplastic materials. The process described thereininvolves making an intimate mixture of a blowing agent with athermoplastic resin and thereafter expanding the mixture to form acellular thermoplastic body. The specification refers to the use ofcycles of expanding operations that, if used repeatedly, cause furtherexpansion of the prepuff particles made in accordance therewith.However, the '386 patent nowhere provides any general statement as tohow many premolding expansion steps should be utilized.

The '386 patent nowhere provides any general description of the amountof blowing agent to be used in the process. However, the '386 patentdoes state that the preferred primary blowing agent isdichlorodifluoromethane, and the Examples in the '386 patent all utilizeonly halogenated methanes as the blowing agent. Example 1 of the '386patent utilizes 2 weight percent dichlorodifluoromethane with eightexpansion steps to achieve an undisclosed final product density. Example2 utilizes 8.11 weight percent dichlorodifluoromethane with nineexpansion steps to effect a 150×volumetric increase (the density of thefinal product was undisclosed). Example 4 utilized 20 volume percent ofdichlorodifluoromethane in a three step expansion process, to effect afinal product density of 0.938 lb./cu. ft. In comparison with theprocess of the present invention, these examples (as well as theremaining examples of the '386 patent) utilize such a high level ofblowing agent (and/or such a high number of expansion steps) that theprocess of the present invention is not only not suggested, the processof the present invention is also taught away from.

The '386 patent also fails to provide one with the unexpected result ofthe present invention: i.e. that if one were to use merely from 2 to 4.4weight percent of a blowing agent, a product of relatively low density(i.e. from 0.8 to about 2 lb./cu.ft.) could be produced with only from 2to 5 expansion steps. The gist of the '386 patent is the generalizednotion that multiple expansion steps can be used to effectuate avolumetric increase greater than the theoretical volumetric increasepossible from the expansion of the blowing agent alone. As the '386patent states repeatedly, this increase is brought about allowing a morepermeable secondary blowing agent, such as air, to diffuse into the foamwhereupon after cooling an additional heating will produce furtherexpansion due to the presence of this secondary blowing agent in furtherheating/expansion steps. Although the process of the present inventioncertainly utilizes this mechanism of increasing the degree of expansion,the process of the present invention is directed towards a specific areawherein this mechanism is used in addition to other critical processsteps, i.e. the use of a low (2-4.4 weight percent) level of blowingagent, in combination with the use of only 2 to 5 expansion steps, aswell as the use of a specific polymer type (i.e. a polymer exhibitingthree characteristics: (1) a polydispersity of from about 1 to less than2.5; (2) a weight average molecular weight of from greater than 180,000to about 300,000; (3) a M_(z) :M_(n) of from about 2 to about 4.5; and(4) branched to from 0 to 5 weight percent) It should be noted that the'386 patent nowhere discloses a polymer having such characteristics.

Furthermore, the process of the present invention isolates a specificarea of improvement over the subject matter disclosed in the '386patent. The process of the present invention relates to the use of beadsof thermoplastic polymer which contain only from about 2 weight percentto about 4.4 weight percent of a hydrocarbon blowing agent. It hassurprisingly been found that even with such a small amount of theblowing agent, the bead can be expanded to a final density of from about0.8 to about 2 lb./cu. ft., while using only 2 to 5 expansion steps (or2 to 4 preexpansion steps before the molding step). The '386 patentnowhere achieves such final product densities while utilizing so littleblowing agent. Finally, the process of the present invention involvesthe release of blowing agent during expansion, aging, and molding, in anamount of from only about 1 weight percent to about 2.5 weight percent.The '386 patent nowhere makes any mention of the amount of blowing agentreleased into the environment.

Applicants have discovered that the use of a low amount of blowing agentprovides many important advantages, among which are:

(1) a reduction in the amount of blowing agent required, resulting incost savings;

(2) reduced environmental pollution since less blowing agent (generallya volatile organic compound, VOC) is released into the environment bothduring manufacture and during consumer use;

(3) processing advantages such as:

(a) a lower shrinkage in the molding step;

(b) quicker cooling in the molding step, resulting in shorter processingtimes;

(c) shorter aging time between the preexpansion steps and between thelast preexpansion step and the molding step, resulting in shorterprocessing times.

U.S. Pat. No. 4,839,396 describes a process for making expandablealkenyl aromatic polymer particles. These particles have the ability touse a decreased amount of blowing agent while maintaining the potentialto produce a bulk density equivalent to that achieved by particlescomprising a greater amount of blowing agent. This is achieved throughthe use of from 0.005 to 0.5 weight percent of a "density modifier". The'396 patent describes the density modifier as a compound providingthermal stability for the alkenyl aromatic polymer at extrusion andexpansion conditions and which is also a liquid plasticizer at expansionconditions. These density modifiers are stated to include octadecyl3,5-di-tert-butyl-4-hydroxyhydrocinnamate, as well as ethylenebis(oxyethylene)bis(3-tert-butyl-4-hydroxy-5-methylhydrocinnamate).

The specification of the '396 patent states that the "volatile fluidfoaming agents" (i.e. the blowing agents) usually are employed inamounts corresponding to from about 5 to about 15 percent of the weightof the total formulation. The only examples in the '396 patent utilizefrom 8.8 to 10.3 weight percent of the blowing agent. These examplesshow that the amount of the blowing agent was reduced from a level of10.0-10.3 weight percent down to about 8.8 weight percent (i.e. areduction of about 12 to 15 percent in the amount of blowing agentused), while achieving the same final density as in the comparative runhaving the greater amount (i.e. 10.0-10.3 weight percent) of blowingagent present.

The process of the present invention also has as a goal the reduction inthe amount of blowing agent used in the manufacture of an expandedpolymer. However, relative to the '396 patent, the process of thepresent invention permits at least as much as twice the percentagereduction in the amount of blowing agent (e.g. most preferably areduction of from about 6 weight percent to about 3.5 weight percent,which is approximately a 40 percent reduction). The process of thepresent invention produces this comparatively large reduction in blowingagent via an approach which is different from the approach taken in the'396 patent. This approach is the use of an initially low level of theblowing agent (i.e. a blowing agent level of from 2 to 4.4 weightpercent) while simultaneously using multiple preexpansion steps. Itshould be noted that although the '396 patent suggests the use ofmultiple preexpansion steps before the molding step, the '396 patentfails to make any connection between the use of low amounts of blowingagent and the use of multiple preexpansion steps, and the '396 patenthas a very broad, undirected disclosure of the amount of blowing agentwhich can be used. Furthermore, the '396 patent suggests the use of onlyrelatively high amounts of blowing agent (i.e. 5 to 15% broadly, withexamples limited to from 8.8% to 10.3% by weight). Finally, the '396patent makes no mention of the amount of blowing agent released duringthe process, unlike the process of the present invention.

U.S. Pat. Nos. 4,520,135 and 4,525,484 (a divisional of the applicationfiled for the '135 patent) are both directed at a particles ofpolystyrene containing a blowing agent, wherein the polystyrene has animproved expandability. More particularly, these particles ofpolystyrene are comprised of a polymer which has a molecular weight offrom about 130,000 to about 180,000. The '135 and '484 patents describeseveral methods for making this polymer, i.e via the use of chaintransfer agents, the use of oligomers, or polymerizing in the presenceof the blowing agent. The '135 and '484 patents state that the resultingpolystyrene particles can be expanded by conventional methods (e.g.steam expansion). The '135 and '484 patents state that the blowing agentcan be present in an amount generally from 3 to 12 weight percent(preferably from 5 to 8 weight percent, and the only example in the '135and '484 patents utilizes approximately 7 weight percent pentane).However, neither the '135 patent nor the '484 patent makes any statementregarding the use of multiple premolding expansion steps.

In contrast, the process of the present invention utilizes a polymerwhich is different from the polymer described in the '135 and '484patents in that it exhibits three characteristics which are differentfrom the polymer described in the '135 and '484 patents. Foremost amongthese differences is the fact that the weight average molecular weightof the polymer used in the process of the present invention is higherthan that of the polymer described in the '135 and '484 patents.Surprisingly, this higher molecular weight polymer has a degree ofexpandability at least as high as the polymer described in the '135 and'484 patents. In further contrast, the process of the present inventionrequires the use of from 2 to 4 premolding expansion steps, whereas the'135 and '484 patents make no mention of multiple expansion steps eitherwith or without molding. Finally, the neither the '135 nor the '484patents make any mention of the amount of blowing agent released duringthe process, unlike the process of the present invention, which callsfor the release blowing agent in an amount of from only about 1 to about2 weight percent.

U.S. Pat. No. 4,485,193 describes a process for making resilient foamparticles and moldings with a "lightly crosslinked" polymer, which couldbe a styrene polymer. The process entails the use of a volatile fluidfoaming agent that has low permeability through the polymer, and theprocess also uses multiple expansion steps for the production of foamsof low density, i.e. suitable for molding. The process described andclaimed in the '193 patent requires that the once-expanded particles aresubjected to a superatmospheric pressure of at least 3 atmospheres inair, whereafter the now "pressurized" particles are further expanded byheating the particles above the glass transition temperature of thepolymer after the particles are returned to a normal atmosphericpressure.

The '193 patent nowhere provides any generalized description of theamount of blowing agent which may be employed in the process describedtherein. However, of the 43 Samples discussed in the '193 patent:Samples 1-17 contained blowing agent in an amount of from about 20 toabout 30 weight percent; Samples 18-21 were foams made "in accord" withthe specification of the '193 patent but no data was provided re theamount of blowing agent employed in the making of the foam; Sample 22was a foam made in accord with the conditions of Sample 2 (and Sample 2utilized 28.9 weight percent blowing agent); Samples 23-38 containedblowing agent in an amount of from 6.12 to about 7.0 weight percent;Samples 39-41 do not contain any description of the amount of blowingagent, but merely state that blowing agent was permitted to diffuse intothe already-formed beads; and Samples 42 and 43 appear to utilizeblowing agent in an amount of at least 11 weight percent In summary, the'193 patent teaches the use of blowing agent in an amount which isconsiderably higher than the amounts involved in the present invention.Finally, the '193 patent makes no mention of the amount of blowing agentwhich is released into the environment.

U.S. Pat. No. 3,639,551 describes a cyclic method for producinglow-density polystyrene foam beads, wherein the beads are expanded in aplurality of expansion steps. However, the gist of the '551 patent isthat in between the expansion steps the now partially-expanded beads arereheated to restore the majority of the "lost volume" (i.e. the volumelost upon the cooling of the beads immediately after they wereexpanded). This reheating step precedes the next expansion step. By thismethod, the shrinkage of the beads will be prevented from substantiallyaffecting the ultimate degree of expansion obtained.

The '551 patent nowhere provides any discussion of the quantity ofblowing agent to be employed in expanding the beads. The '551 patent hasa single example which states that:

1000 pounds of a commercial grade acrylonitrile styrene copolymer beadshaving low boiling hydrocarbon propellant (pentane) included therein anda diameter between about 1/64 to about 1/32 inch were stored in a hopperand fed into a "Buccaneer" preexpander (available from TRI Manufacturing& Sales Co., Lebanon, Ohio) having an expansion chamber substantially asdescribed above.

The '551 patent nowhere states how much of the low boiling propellant(pentane) was present in the beads.

In stark contrast to the '551 patent, the process of the presentinvention utilizes a low level of blowing agent (from about 2 to about4.4 weight percent) in combination with multipass expansion in order toachieve a product having a density of from about 0.8 to 2 pounds percubic foot. The '551 patent nowhere mentions the use of such anunconventionally low amount of blowing agent. The process of the presentinvention further requires the use of a specific polymer, which polymerthe '551 patent nowhere discloses. Finally, the process of the presentinvention involves the release of a specified amount of blowing agent,while the '551 patent nowhere mentions the amount of blowing agentreleased during the process.

U.S. Pat. No. 3,631,133 describes a process for expanding polystyrenebeads in order to produce a bead having an exceptionally low density(i.e. 5 kg./cu. meter, or less, which equals approximately 0.3 lb./cu.ft., or less). The method described in the '133 patent is generallydescribed as:

(1) insufflating (i.e. pre-expanding to produce a partly expandedproduct) polystyrene granules containing blowing agent, the insufflatingbeing carried out with steam at about atmospheric pressure, whereby thegranules are partially expanded;

(2) conditioning the partially expanded granules at atmosphericpressure;

(3) subjecting the partially expanded granules (in a confined space) tosteam at about 150 g./sq. cm. pressure; and

(4) restoring the expanded granules to atmospheric temperature andpressure.

The gist of the '133 patent is that of using multiple expansion steps incombination with a conditioning step, in order to ultimately produce alow density product. The '133 patent nowhere refers to the quantity ofblowing agent to be utilized in the process. Rather, all thespecification (including Examples) has to say about the blowing agentis:

The starting material consisting of granules of polystyrene containing(sic) a pentane petroleum fraction as a blowing agent. [Col. 2, lines61-63]

In stark contrast to the '133 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 2 pounds per cubic foot. The '133 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '133 patent nowhere refers to the characteristics ofthe polystyrene polymer used therein as providing anything other than aconventional level of expandability.

In stark contrast to the '133 patent, the process of the presentinvention further specifies the release of blowing agent in an amount offrom only about 1 to about 2 weight percent. The '133 patent nowherementions the amount of blowing agent released.

U.S. Pat. No. 3,598,769 describes a process for expanding polystyrene,this process involving:

(1) subjecting (for a few minutes) polystyrene granules to steam at lowpressure;

(2) conditioning the granules for a few hours at about 20° C. to 40° C.;

(3) reheating the expanded granules to about 100° C. with hot air;

(4) then treating the granules with steam for 30 to 40 seconds; followedby

(5) conditioning the granules for 1 to 24 hours.

The objective of the '769 patent is to provide a process for producingbeads of polystyrene having an apparent specific mass less than about 7kg./cu. meter (i.e. a density of about 0.44 lb/cu.ft., or less). Thegist of the '769 patent is to provide a very specific process for usingtwo of expansion steps and a conditioning step after each expansionstep. Furthermore, the '769 patent is directed at carrying out thisprocess on a continuous conveyor belt. As with the '133 patent, the '769patent nowhere describes either the amount of blowing agent to be usedin the process or the amount of blowing agent emitted during theprocess.

In stark contrast to the '769 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 2 pounds per cubic foot. The '769 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '769 patent nowhere mentions either the use of apolymer having an extraordinary degree of expandability or the emissionof blowing agent in an amount of from about only 1 to about 2 weightpercent.

U.S. Pat. No. 3,126,432 describes a process for producing super-lowdensity thermoplastic foam, namely polystyrene foam. The processdescribed in the '432 patent involves expanding particles of polystyrenehaving a vaporizable liquid (butane or pentane) inflating agent therein,and thereafter aging the expanded particles first at atmosphericpressure and thereafter at superatmospheric air pressure (2 to 8atmospheres) for several hours. This exposure to superatmospheric airpressure has the effect of causing a secondary blowing agent to migrateinto the expanded particles. Thereafter, the pressure is released andwithin five hours the particles are heated in a closed mold. Thus thegist of the '432 patent is to "pump up" the expanded particles byexposing the particles to superatmospheric air, and thereafter carryingout a second expansion step by taking advantage of the relatively highinternal pressure within the particles, once they are released from thepressure chamber.

The '432 patent nowhere has any general discussion of the amount ofblowing agent to be utilized in making polystyrene foams. Of the fiveexamples given in the '432 patent, only Examples I, III, and V provideany information as to the amount of blowing agent used in the process.In each of these Examples, the blowing agent used is pentane, and thepentane is present in an amount of 6% by weight of the polystyreneglobules. Thus it is clear that the '432 patent does not teach towardsany process which utilizes a blowing agent in an amount less than 6% byweight.

In stark contrast to the '432 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 2 pounds per cubic foot. The '432 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '462 patent nowhere mentions a polymer exhibiting theproperties of the present invention. Finally the '432 patent nowherementions the amount of blowing agent emitted during the processdescribed therein, unlike the process of the present invention.

U.S. Pat. No. 3,056,753 describes the production of expandable polymericparticles having a foamed polymeric structure. The process describedtherein involves:

(1) partially expanding the polymeric particles; followed by

(2) crushing the particles; followed by

(3) again partially expanding the particles.

The gist of the '753 patent is to decrease the molding cycle time,whereby molded articles can be removed from the mold after permitting acooling period of lower duration. Nowhere in the '753 patent is thereany mention of the amount of blowing agent to be utilized in theprocess. In fact, even the seven examples within the '753 patent fail toprovide any information as to the amount of blowing agent utilized.

In stark contrast to the '753 patent, the process of the presentinvention utilizes a low level of blowing agent in combination withmultipass expansion in order to achieve a product having a density offrom 0.8 to 2 pounds per cubic foot. The '753 patent nowhere mentionsthe use of such an unconventionally low amount of blowing agent.Furthermore, the '753 patent nowhere mentions a polymer having thecharacteristics of the polymer of the present invention. Finally, the'753 patent nowhere mentions the amount of blowing agent emitted duringthe process, unlike the process of the present invention.

U.S. Pat. No. 4,721,588 describes a closed circuit process for theproduction of expanded polystyrene foam. This process comprises thesteps of:

(a) pre-expanding raw polystyrene beads containing a blowing agent in apre-expansion vessel;

(b) storing the beads in one or more closed storage containers to allowthe internal pressure within the expanded beads to return tosubstantially atmospheric pressure;

(c) molding the expanded beads to a desired configuration in a closedmold with steam; and

(d) removing the thus-formed article from the mold and placing such inan aging room, wherein at each stage the blowing agent released from thebeads is recovered, separated from any residual steam by means of acondensing system, and introduced into the burner of a steam generator,thereby serving as fuel for the process.

As can be seen from the above description of steps, the gist of theprocess described in the '588 patent is the recovery of the blowingagent and its re-use as a fuel for the heating step. This produces thedual effects of (1) reducing the amount of volatile organic compoundsreleased into the atmosphere, as well as (2) obtaining a double use forthe blowing agent which escapes from the polystyrene during thepre-expansion and molding steps.

The '588 patent also mentions that the blowing agent can be n-pentane,or mixtures of n- and iso-pentane (up to about 25% iso-pentane byweight) The '588 patent also states that the initial blowing agentcontent of the expandable polystyrene beads can be 4-8 weight percentHowever, the '588 patent nowhere states that a low density foamedpolystyrene product can be obtained if one utilizes less than 5 weightpercent of the blowing agent. In fact, aside from the statement that ablowing agent content of 4-8 weight percent can be used, the '588 patentmakes absolutely no mention of actual product densities or any furthermention of the amount of blowing agent in any actual composition.Finally, the '588 patent makes no mention of the use of multiplepre-molding expansion steps. Rather, the '588 patent teaches one tosimply perform one pre-molding expansion step and to thereafter followthis step with the molding step. Although the '588 patent is concernedwith preventing the release of emitted blowing agent into theenvironment, the '588 patent nowhere describes the amount of blowingagent released during the expansion and molding process, unlike theprocess of the present invention.

In contrast to the '588 patent, the process of the present inventionutilizes a low level of blowing agent in combination with multipassexpansion in order to achieve a product having a density of from 0.8 to2 pounds per cubic foot. The '588 patent nowhere mentions the use of thecombination of a level of blowing agent less than 5 weight percent withmultipass expansion in order to achieve a product density of from 0.8 topounds per cubic foot. Furthermore, the '588 patent nowhere mentions apolymer having the characteristics of the polymer used in the process ofthe present invention. Finally, the '588 patent nowhere mentions theamount of blowing agent emitted during the expansion process, unlike theprocess of the present invention.

In recent years emissions of volatile organic compounds (VOCs) have comeunder increasing scrutiny by the EPA, state, and local air qualityboards as mandated by the Clean Air Act of 1977. Because hydrocarbonemissions have been shown to contribute to photochemical smog, theexpanded polystyrene industry (which uses pentane as a blowing agent)has come under pressure to limit its use and/or emissions of pentane.

Since the early months of 1990, the inventors' process has enjoyed ahigh level of commercial success, with sales of at least 2 milliondollars of a formulation which has been expanded only with theinventors, process (which corresponds with sales of approximately 3million pounds of the formulation), which formulation has ahighly-expandable polymer present in an amount of about 96 weightpercent, based on the total weight of products. Thus there has been ahigh level of commercial success of both the process as well as theformulation utilized in practicing the process.

For several years BASF Corporation has been involved in the manufactureand sale of a polystyrene product having approximately 6 weight percentpentane therein. This product had a polydispersity of 2.2, a weightaverage molecular weight of about 190,000, and an M_(z) :M_(n) of about3.5. In stark contrast, the product of the present invention has apolydispersity of from 1 to less than 2, a weight average molecularweight of from about 200,000 to about 300,000, and an M_(z) :M_(n) offrom about 2 to less than 3.

One polymer which has been commercialized for several years has apolydispersity of about 1.9, a weight average molecular weight of about190,000, and furthermore, upon analysis, yielded an Mz:Mn of 3.04.Furthermore, this polymer was produced only in formulations bearingblowing agent in an amount of about 6 weight percent. In contrast, thepolymer of the present invention has a combination of characteristics(polydispersity, weight average molecular weight, and M_(z) :M_(n)),which differs from the aforementioned commercially available polymer.Furthermore, the formulation of the present invention utilizes blowingagent in an amount of only from about 2 weight percent to about 4.4weight percent.

BRIEF SUMMARY OF THE INVENTION

The process of the present invention pertains to a process for making anexpanded polymeric product while emitting only a small amount of avolatile blowing agent. The process is carried out by expanding beads infrom 2 to 5 expansion steps. The beads are comprised of a blowing agentand a polymer. The blowing agent is homogeneously dispersed in thepolymer and the blowing agent may be, in general, hydrocarbons which aregaseous or liquid under normal conditions, do not dissolve the styrenepolymer, and boil below the softening point of the polymer. The blowingagent is preferably at least one member selected from the groupconsisting of:

pentane, cyclopentane, methylcyclopentane, neopentane, isopentane,pentane petroleum distillate fractions, propane, butane, isobutane,hexane, isomers of hexane, 2-methyl pentane, 3-methyl pentane,2,2-dimethylbutane, 2,3-dimethylbutane, cyclohexane, methylcyclohexane,heptane, propylene, 1-butylene, 2-butylene, isobutylene, mixtures of oneor more aliphatic hydrocarbons having a molecular weight of at least 42and a boiling point not higher than 95° C. at 760 millimeters absolutepressure, water, carbon dioxide, ammonium carbonate, and azo compoundsthat are decomposable to form a gas at a heat-plastifying temperature towhich the polymer is brought.

The blowing agent is present in the beads in an amount of from about 2weight percent to about 4.4 weight percent based on the weight of thebeads.

The polymer making up the beads may be one or more polymers producedfrom at least one of a variety of monomers. The monomer is at least onemember selected from the group consisting of:

styrene, derivatives of styrene, vinyltoluene, mono- and polyhalogenatedvinyltoluenes which form linear polymers, acrylonitrile, and methylmethacrylate.

The polymer is present in the beads in an amount of from about 93 weightpercent to about 98 weight percent based on the weight of the beads. Thepolymer exhibits the following three characteristics: (1) apolydispersity of from about 1 to less than 2.5; (2) a weight averagemolecular weight of from greater than about 180,000 to about 300,000;and (3) an M_(z) :M_(n) of from about 2 to about 4.5. Furthermore, thepolymer is branched to from 0 to less than 5 weight percent.

The expansion steps are carried out in an expander at substantiallyatmospheric pressure. The expansion steps result in "finally-expanded"beads. The expansion of the polystyrene beads is carried out in a mannerso that the finally-expanded beads have a density of from about 0.8pounds per cubic foot to about 1.1 pounds per cubic foot.

Furthermore, the process is carried out so that the blowing agentemitted in the expansion steps is only from about 0.3 to about 1.5weight percent based on the weight of the beads. The expansion stepsresult in "finally-expanded" beads. The expansion of the beads iscarried out in a manner so that the finally-expanded beads have adensity of from about 0.8 pounds per cubic foot to about 2 pounds percubic foot.

The process of the present invention also pertains to a process formaking a closed-cell foamed thermoplastic resinous molded object. Theprocess is carried out by preexpanding the beads in from 2 to 4preexpansion steps, and thereafter carrying out a molding step in whichthe beads are further expanded and fused into a unitary object, whileemitting blowing agent in an amount of from only about 1 to about 2.5weight percent. In the process in which there is a molding step, theexpansion steps which precede the molding step are termed "preexpansion"steps because they precede the molding step. It should be noted that themolding step generally causes at least some further expansion, alongwith bonding the preexpanded beads to one another (i.e. fusion).

The beads are comprised of a blowing agent and a polymer, with thepolymer being present in an amount of from about 93 weight percent toabout 98 weight percent, and the blowing agent being present in anamount of from about 2 to about 4.4 weight percent. The blowing agent isat least one member selected from the group consisting of:

pentane (including isomers of pentane such as cyclopentane, neopentane,isopentane, as well as pentane petroleum distillate fractions),methylcyclopentane, propane, butane, isobutane, hexane, isomers ofhexane, 2-methyl pentane, 3-methyl pentane, 2,2-dimethylbutane,2,3-dimethylbutane, cyclohexane, methylcyclohexane, heptane, propylene,1-butylene, 2-butylene, isobutylene, mixtures of one or more aliphatichydrocarbons having a molecular weight of at least 42 and a boilingpoint not higher than 95° C. at 760 millimeters absolute pressure,water, carbon dioxide, ammonium carbonate, and azo compounds that aredecomposable to form a gas at a heat-plastifying temperature to whichthe polymer is brought.

The polymer making up the beads may be one or more polymers producedfrom at least one of a variety of monomers. The monomer is at least onemember selected from the group consisting of:

styrene, derivatives of styrene, vinyltoluene, mono- and polyhalogenatedvinyltoluenes which form linear polymers, phenyl ether, acrylonitrile,and methyl methacrylate.

The polymer is present in the beads in an amount of from about 93 weightpercent to about 98 weight percent based on the weight of the beads. Thepolymer exhibits the following three characteristics: (1) apolydispersity of from about 1 to less than 2.5; (2) a weight averagemolecular weight of from greater than about 180,000 to about 300,000;and (3) an M_(z) :M_(n) of from about 2 to about 4.5. Furthermore, thepolymer is branched to from 0 to less than 5 weight percent.

The completion of the preexpansion steps results in the production of"finally-preexpanded" beads. The finally-preexpanded beads are thenmolded in order to further expand them and fuse them together into aunitary, molded expanded polymeric product. Both the preexpansion stepsand the molding step are carried out so that a molded foamed objecthaving a density of from about 0.8 to about 2 pounds per cubic foot isformed.

It is an object of the present invention to reduce the amount of blowingagent used in the production of closed cell, foamed thermoplasticresinous objects.

It is a further object of the present invention to reduce the level ofenvironmental impact in the production of closed cell, foamedthermoplastic resinous objects, by reducing the amount of emissions ofblowing agents released into the environment.

It is a further object of the present invention to provide a processwhich will lower the shrinkage upon molding in the production of closedcell, foamed thermoplastic resinous objects.

It is a further object of the present invention to provide a processwhich results in a lowering of the required cooling times both betweenexpansion (and preexpansion) steps as well as in any molding stepmolding step which is utilized.

It is a further object of the present invention to reduce the amount ofblowing agent used in the production of low density, closed cell, foamedthermoplastic resinous objects.

It is a further object of the present invention to reduce the level ofenvironmental impact in the production of low density, closed cell,foamed thermoplastic resinous objects.

It is a further object of the present invention to provide a process forefficiently using a polystyrene polymer having a high degree ofexpandability.

It is a further object of the present invention to provide a process forutilizing an expandable polystyrene formulation in the production ofexpanded polystyrene products.

It is a further object of the present invention to provide a processwherein a polystyrene polymer as well as a formulation for makingexpanded polystyrene products can be utilized with a lesser amount ofblowing agent emitted, so that there is less blowing agent forsubsequent capture in pollution abatement equipment.

It is a further object of the present invention to provide a process forproducing expanded polystyrene products while using a formulation havinga greater ratio of polymer to blowing agent, so that more polymer ispresent per pound of formulation.

It is a further object of the present invention to enable the productionof expanded polystyrene products using decreased molding cycle times, aswell as decreased shrinkage upon molding, as well as decreased agingtimes between expansion steps.

It is a further object of the present invention to enable the productionof an expanded polystyrene product having decreased susceptibility todamage during processing.

It is a further object of the present invention to enable the productionof an expanded polystyrene beads having increased shelf life before themolding step due to a lower rate of loss of blowing agent therefrom.

It is a further object of the present invention to enable a process formaking expanded polystyrene products in which there is decreasedsensitivity to steam during the expansion and molding steps, therebypermitting a "broader molding range" process with respect to the use ofsteam in the preexpansion and molding steps, and its effect on cycletime, fusion, and dimensional stability upon molding.

Many of the above objects can further be understood as providing arespective advantage to the process of the present invention. Althoughthe term "polystyrene" is found in the above object statements, theseobjects should be understood as being applicable to all polymers whichmay be utilized in the process of the present invention, as well as apreferred applicability to polystyrene.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process of the present invention it is an objective to provide acommercially viable process for making expanded polymeric products inwhich there is a reduced level of emissions of volatile organiccompounds in comparison with currently viable commercial processes. Inpart, this objective is achieved by utilizing a lower amount of blowingagent than has been used in prior art commercially viable processes. Theprocess is carried out by using from 2 to 4.4 weight percent of theblowing agent, based on the weight of the polymer, together with apolymer which has a high degree of expandability. The inventors of theprocess of the present invention have unexpectedly discovered that sucha polymer can also be used to make a low density (i.e. 0.8-1.1lb./cu.ft.) foam while using an unexpectedly low amount of blowingagent, if from just two to five expansion steps are utilized in theprocess.

In general in the process of the present invention in which there is nomolding step, blowing agent is emitted in an amount of from about 0.3weight percent to about 1.5 weight percent, based on the total weight ofthe beads. Preferably the emission of blowing agent is from about 0.5 toabout 1.2 weight percent, and most preferably the emission of blowingagent is from about 0.6 to about 0.7 weight percent. These emissionfigures represent the sum total of blowing agent which is released tothe environment during the expansion steps themselves as well as each ofthe aging periods between each of the expansion steps.

In general in the process of the present invention in which there is amolding step, blowing agent is emitted in an amount of from about 1weight percent to about 2.5 weight percent, based on the total weight ofthe beads. Preferably this emission of blowing agent is from about 1.5to about 1.9 weight percent, and most perferably this emission ofblowing agent is from about 1.6 to about 1.8 weight percent. Theseemission figures represent the sum total of blowing agent which isemitted during the preexpansion steps themselves plus each of the agingperiods after each preexpansion step, plus the emissions produced by themolding step.

The process of the present invention involves expanding a substantiallysolid thermoplastic polymer to form a foam. The expansion of the polymeris effectuated by intimately mixing a blowing agent with the polymer,and thereafter heating the mixture so that the blowing agent vaporizeswithin the polymer particles, causing the polymer particles to expandduring a period in which the polymer is in a softened state. Thevaporization of the blowing agent is produced by the application ofheat. Likewise, the heat also softens the polymer. Enough heat must beapplied to cause the temperature of the polymer to exceed its softeningpoint. The vaporization of the blowing agent within the softened polymercauses the mixture to expand and form a foam. The foam is then allowedto cool, while remaining substantially expanded.

During cooling, the pressure within the foam cells decreases due tocooling and condensing of the blowing agent. This causes gases which canpermeate the polymer (e.g. air, steam, etc.) to migrate into the cells,thereby somewhat restoring (i.e. to atmospheric pressure) the relativelylow internal pressure within the cells. Although components within theatmosphere (i.e. oxygen, carbon dioxide, nitrogen, etc) are to somedegree able to diffuse into the cells, if steam is used as the source ofheat for the expansion steps (or the molding step), it generally is themost permeable of the gases diffusing into the cells of the foam. Uponsubstantial equilibration (i.e. when the pressure within the cells issubstantially that of ambient atmospheric pressure) of the foam, the nowcooled foam can again be heated, resulting in further expansion of thefoam. Thus by utilizing multiple "cycles", or "passes" of suchexpansion, cooling, and "aging" (i.e. substantial equilibration ofpressure), sequential volumetric increases can be achieved.

Optionally, the foam can be further expanded and fused in a moldingstep, in order to form a molded object. Molding is effectuated byplacing preexpanded beads into a mold, closing the mold so that asubstantially confined volume is produced, and thereafter furtherheating the preexpanded beads so that they further expand andsubstantially fill the volume within the mold and fuse (i.e. bond) toone another.

In the process of the present invention it is an objective to provide acommercially viable process which reduces the emission of volatileorganic compounds in comparison with currently viable commercialprocesses. In part, this objective is achieved by utilizing a loweramount of blowing agent than has been used in prior art commerciallyviable processes. This is effectuated by using from 2 to 4.4 weightpercent of the blowing agent, based on the weight of the polymer.

Any one or more of a wide variety of blowing agents can be utilized inthe process of the present invention. These blowing agents includehydrocarbons which are gaseous or liquid at standard temperature andpressure, do not dissolve the styrene polymer, and boil below thesoftening point of the polymer.

Among the blowing agents preferred for use in the process are, forexample: pentane (including isomers of pentane such as cyclopentane,neopentane, isopentane, as well as pentane petroleum distillatefractions), methylcyclopentane, propane, butane, isobutane, hexane,2-methyl pentane, 3-methyl pentane, cyclohexane, methylcyclohexane,heptane, propylene, 1-butylene, 2-butylene, or mixtures of one or morealiphatic hydrocarbons having a molecular weight of at least 42 and aboiling point not higher than 95° C. at 760 millimeters absolutepressure, and azo compounds that are decomposable to form a gas at aheat-plastifying temperature to which the polymer is brought. A morepreferred group of blowing agents comprises pentane, cyclopentane,neopentane, isopentane, pentane petroleum distillate fractions, propane,butane, isobutane, hexane, cyclohexane, and heptane.

The blowing agent may be incorporated into the polymer before, during,or after polymerization. In the process of the present invention, theblowing agent is incorporated into the polymer in an amount of from 2 to4.4 weight percent, based on the total bead weight. Preferably theblowing agent is incorporated in an amount of from about 2.5 to about4.4 weight percent, still more preferably from about 3 to about 4, andmost preferably the blowing agent is incorporated in an amount of about3.5 weight percent. Most preferably the blowing agent is pentane (eitherpure pentane or a mixture of n-pentane together with isomers ofpentane). Preferably the pentane is added to the polymer during thepolymerization process, most preferably at a point in the polymerizationprocess at which the level of styrene conversion is from about 20percent to about 60 percent.

As stated above, the process of the present invention is carried out onan unexpanded bead. According to the process of the present invention,unexpanded beads are expanded in from 2 to 5 expansion steps. Thepreferred process of the present invention involves carrying out 2 or 3preexpansion steps, followed by a molding step. That is, from 2 to 3premolding expansion steps are utilized to produce finally-preexpandedbeads. Most preferably this process utilizes only 2 preexpansion steps.

If the process is being utilized to make a non-molded product, thepreferred process is to use from 2 to 3 expansion steps to producefinally-expanded beads. Most preferably this process utilizes only 2expansion steps.

If the beads are to be expanded and not fused into a molded object, thebeads are herein termed as "finally expanded beads". If, however, thebeads are ultimately to be fused into a molded object, the expansionsteps are termed "preexpansion steps". Once these beads are "finallypreexpanded", they are thereafter further expanded and fused into amolded object in a molding step which also serves as a final "expansion"step. Regardless of whether the beads are simply expanded or arepreexpanded and thereafter molded, according to the process of thepresent invention the total number of expansion (including preexpansion)steps is no greater than 5 and no less than 2, and if there is a moldingstep there are at least 2 preexpansion steps. It has surprisingly beenfound that with as little as from 2 to 4.4 weight percent of blowingagent, a final product density as low as from 0.8 to 1.1 lb/cu.ft. canbe obtained. Foams having this density are useful as insulation orpackaging.

The expansion of the beads is typically carried out in a batch expanderclosed vessel having steam injected thereinto. Examples of suchexpanders include: Tri 502, Tri 905, Weiser VN400, Kurtz KV1000,Dingledein VA2000. The expansion of the beads is carried out by passingthe beads through an expander so that the beads are heated and becomesoft enough that they expand due to the rising pressure produced by thevaporization and subsequent expansion of the blowing agent. As a generalrule, the rate of passage of the beads through the expander determinesthe amount of expansion which will result during that expansion step. Ofcourse, the lower the rate of passage of beads through the expander, thegreater the amount of heat transferred to the beads, and the higher theresulting degree of expansion produced. However, there is a maximumamount of expansion which any one expansion step can produce for anygiven bead composition. Thus, it has been found that in general the flowrate of the beads through the expander should be a rate of passagethrough the expander of from about 5 pounds per hour per cubic foot ofexpander volume (lb/hr/cu.ft.) to about 120 lb/hr/cu.ft. At flow ratesbelow 5 lb/hr/cu.ft , the beads remain in the expander so long thatexcessive lumping occurs. At flow rates above 120 lb/hr/cu.ft., thebeads do not remain in the expander long enough to cause sufficientexpansion to result in a process of expanding with a reasonable degreeof efficiency. Preferably, the expansion rate is from about 7 to about100 lb/hr/cu.ft., and most preferably the expansion rate is from about12 to about 80 lb/hr/cu.ft.

The process of the present invention can be carried out either with orwithout a molding step. The following is a description of the moldingstep which can be utilized in the process:

The prepuff (i.e. the preexpanded beads) were placed into a Kurtz vacuumblock mold of internal dimensions of approximately 48"×96"×33". Themolding steps were as follows: presteaming vacuum to approximately 0.5bar absolute pressure, followed by steaming into vacuum forapproximately 3 seconds, then cross-steaming through the block for about3-6 seconds, then autoclaving for another 3-8 seconds to a maximum foampressure of approximately 0.5-1.0 bar. Vacuum was then applied to theblock in the mold to assist in cooling the block to a foam pressure ofapproximately 0-0.1 bar, allowing the block to be removed from the moldwithout significant post-expansion occurring.

The process of the present invention also may optionally utilizepoly-n-vinylpyrollidone as a protective colloid which coats the beads.The poly-n-vinylpyrollidone is added at a time during the polymerizationreaction when the beads are at the size desired. Thepoly-n-vinylpyrollidone has the effect of coating the beads so that theycannot adhere to one another, resulting in arresting the growth of thebeads, thereby "freezing" their size. The poly-n-vinylpyrollidone ispreferably added to the reaction mixture at a level of about 0.3 weightpercent, based on the weight of the polymer present.

Generally, the polymers to be used in the process of the presentinvention may be produced from one or more of any of a wide variety ofmonomers. Such monomers include monovinyl compounds which undergoaddition polymerization to provide generally linear polymers. It ispreferred that such polymers are also capable of forming structurescrosslinked to a desired degree when polymerized in the presence of acrosslinking quantity of a polyvinyl compound (e.g. ethylene glycol,dimethacrylate, divinylbenzene, etc.). The monovinyl compounds useful inthe process are, for example, styrene (and derivatives thereof),vinyltoluene, mono- and polyhalogenated vinyltoluenes which form linearpolymers, acrylonitrile, methyl methacrylate, and vinyl toluene.

Preferably the monomer is at least one member selected from the groupconsisting of styrene and derivatives of styrene. Polystyrene is themost preferred polymer for the process of the present invention.Although pure polystyrene is the most preferred styrene polymer for usein the present invention, monomers herein termed "derivatives ofstyrene" which can be polymerized and used in the process of the presentinvention include:

alpha-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene,ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene, and ar-bromostyrene,or solid copolymers of two or more of such alkenyl aromatic compoundswith minor amounts of other readily polymerizable olefinic compoundssuch as divinylbenzene, methylmethacrylate or acrylonitrile, etc.

The process of the present invention requires the use of a polymer whichexhibits the following three characteristics: (1) a polydispersitywithin a given range; (2) a weight average molecular weight within agiven range; and (3) an M_(z) :M_(n) within a given range. The polymerused in the process of the present invention is herein defined in termsof weight average molecular weight (M_(w)), number average molecularweight (M_(n)), "z-average" molecular weight (M_(z)). The number averagemolecular weight is the arithmetic mean value obtained by dividing thesum of the individual polymeric molecular weights by the number ofmolecules present. The weight average molecular weight is the secondpower average molecular weight in the polydisperse polymer. Thez-average molecular weight molecular weight is the third power averagemolecular weight in the polydisperse polymer. More extensive anddescriptive definitions of these various molecular weights weredescribed by Billmeyer, F. W., Jr., Textbook of Polymer Science, 2ndEd., 1971,Wiley-Interscience, N.Y., N.Y., pp 6, 66, 78, and 92, whichbook is herein incorporated by reference.

The process of the present invention utilizes a polymer having aparticular set of characteristics, which characteristics are derivedfrom the molecular weight distribution curve of the polymer. Themolecular weight distribution curve is determined by gel permeationchromatography. This method is described in detail in G. Glockler,Polymercharakterisierung, Chromatographische Methoden, volume 17,published by Huthig, Heidelberg 1982, which is herein incorporated byreference.

The first of these characteristics, i.e. polydispersity, is determinedby analyzing the molecular weight distribution curve for the reactionproduct of the polymerization. Polydispersity is calculated by dividingthe weight average molecular weight by the number average molecularweight. Thus polydispersity is a measure of the breadth of the molcularweight distribution. The polymer generaly exhibits a polydispersity offrom about 1 to less than 2.5. Preferably, the polymer exhibits apolydispersity of from about 1 to less than 2.0, still more preferablyfrom about 1.5 to less than 2.0, and most preferably the polymerexhibits a polydispersity of from about 1.7 to about 1.98. Example 7(infra) discribes the method of analysis of the polymeric reactionproduct, this method providing the means for determintion of weightaverage molecular weight, number average molecular weight, and"z-average molecular weight". Thus this analytical procedure providesthe data from which one may then calculate polydispersity, weightaverage molecular weight, and the M_(z) :M_(n) ratio.

The second characteristic which the polymer exhibits (i.e. the weightaverage molecular weight) is, in general, from greater than about180,000 to about 300,000. Preferably, the polymer used in the process ofthe present invention has a weight average molecular weight of fromgreater than 190,000 to about 250,000. Most preferably, the polymerexhibits a weight average molecular weight of from about 200,000 toabout 220,000. As with polydispersity, the weight average molecular isdetermined the analysis provided in Example 7, infra.

The third characteristic is M_(z) :M_(n), i.e. the ratio of thez-average molecular weight to the number average molecular weight. Thisratio is related to the steepness of slope of the upper end of themolecular distribution curve. In general, the polymer exhibits an M_(z):M_(n) ratio of from about to 2 to about 4.5. Preferably the polymerexhibits an M_(z) :M_(n) ratio of from about 2.5 to about 3.3, and mostpreferably 2.7-3.0. As with polysidpersity and weight average molecularweight, the M_(z) :M_(n) ratio can be calculated based upon the analyticresults obtained from the procedure of Example 7. This procedure, ofcourse, results in obtaining a molecular weight distribution curve. Thevalue for weight average molecular weight, number average molecularweight, and "z-average molecular weight" can be determined. These valuespermit the calculation of polydispersity as well as M_(z) :M_(n) ratio.

The polymer used in the process of the present invention is asubstantially linear polymer, i.e. is a substantially unbranchedpolymer. In general the polymer has a degree of branching of from 0 toless than 5 weight percent. The phrase ". . . branched to from 0 to lessthan 5 weight percent . . . " is herein defined as referring to apolymeric chain in which at least 95 percent of the molecular weight ofthe polymer resides in that portion of the molecule which constitutesthe linear chain. For purposes of calculating the weight percent of thepolymer which resides in branches (as opposed to the linear portion ofthe polymer molecule), carbon atoms which are not part of the mainpolymeric chain are considered to be located on branches, and any atomswhich are attached to the branch carbon atoms are likewise considered tobe located on the branch portion of the polymer molecule. Non-carbonatoms which are bonded to a carbon atom of the linear polymer backbone(but which themselves do not make up a portion of the backbone) areconsidered substituents, rather than branches. However, if a substituentatom is bonded directly or indirectly to a second carbon atom whereinthe second carbon atom is not part of the linear polymer backbone, thesubstituent as well as any atoms attached thereto (which are not part ofthe polymer backbone) are considered to be on a branch. The polymer ispreferably branched to from 0 to less than 2 weight percent. Mostpreferably the polymer is branched to from 0 to less than 1 weightpercent.

The polymer is preferably a substantially homopolymeric polystyrenepolymer. That is, the polymer is preferably derived for a single monmer,that monomer being styrene. The phrase "substantially homopolymericpolymer" is herein defined as a polymer in which at least 99 percent ofthe monomeric units (which reacted to form the polystyrene) were asingle monomer. Preferably, at least 99.9 percent of the monomeric unitswhich are reacted to form the polymer are a single monomer species.

Preferably the polymer is a substantially unsubstituted polymer. Thephrase "substantially unsubstituted polymer" is herein defined as apolymer having a carbon backbone and branches in which less than 2percent of the available sites for substitution have atoms other thanhydrogen thereon. Still more preferably, the degree of substitution isless than 0.5 percent, based on the total number of positions forsubstitution available on the polymer.

A perferred polymer of the present invention exhibits (1) apolydispersity of from about 1.5 to less than 2.0; (2) a weight averagemolecular weight of from greater than 190,000 to about 250,000; and (3)an M_(z) :M_(n) of from about to 2.5 to about 3.3. Furthermore, thispreferred polymer is branched to from 0 to less than 2 weight percent.Finally, this preferred polymer is a substantially homopolymeric,unsubstituted polymer.

A still more preferred polymer of the present invention exhibits: (1) apolydispersity of from about 1.7 to about 1.98; (2) a weight averagemolecular weight of greater than about 200,000 to about 220,000; and (3)an M_(z) :M_(n) of from about 2.7 to less than about 3.0. Furthermore,this still more preferred polymer is branched to from 0 to less than 1weight percent. Finally, this still more perferred polymer is asubstantially homopolymeric, unsubstituted polymer.

Preferably the formulation of the present invention further comprises achain transfer agent. Chain transfer agents having a transfer constant K(as defined in Vollmert, Grundriss der Makromolekularen Chemie,published by Springer 1962, pages 52 and 71, which is herebyincorporated by reference) of from 0.1 to 50, preferably from 1 to 30,are used. Examples of suitable chain transfer agents are:

    ______________________________________                                        n-Dodecyl mercaptan    (K = 19)                                               tert.-Dodecyl mercaptan                                                                              (K = 3)                                                n-Butyl mercaptan      (K = 22)                                               tert.-Butyl mercaptan  (K = 3.6)                                              Carbon tetrabromide    (K = 2.2)                                              Pentaphenylethane      (K = 2.0)                                              ______________________________________                                    

The process of the present invention can optionally employ a flameretardant. The preferred flame retardant is hexabromocyclododecane.Preferably the process of the present invention utilizes a flameretardant in the mixture of components which makes up the formulation ofthe expandable beads. In general, the flame retardant is an organicbromine or chloring flame retardant compound present in an amount offrom about 0.2 to about 2 weight percent, based on the weight of thetotal formulation. More preferably the formulation comprises abrominated hydrocarbon flame retardant in an amount of from about 0.5 toabout 1.5 weight percent, based on the weight of the total formulation.Still more preferably the formulation comprises a flame retardant whichis at least one member selected from the group consisting oftrisdibromo-propylphosphate, hexabromocyclododecane, and bis allyl etherof tetrabromo-bisphenol A and, wherein the flame retardant is present inan amount of from about 0.6 weight percent to abour 1.2 weight percent,based on the total weight of the formulation.

Preferably the formulation further comprises a "flame retardantsynergist", i.e. one or more compounds which increase the effectivenessof the flame retardant when used in combination therewith. The flameretardant synergist may at least one member selected from the groupconsisting of dicumyl peroxide and other organic peroxides which have ahalf-life of 1 hour at temperatures of from about 110° C. to about 150°C.

The formulation may further comprise additional additives which impartparticular properties to the expandable products. Examples include,antistatic agents, stabilizers, colorants, lubricants, fillers,substances which prevent agglomeration during prefoaming, e.g. zincstearate, melamine-formaldehyde condensates or silica, and agents forreducing the demolding time during final foaming, e.g. glycerol estersor hydroxycarboxylic acid esters. Depending on their intended effect,the additives may be homogeneously dispersed in the particles or bepresent as a surface coating.

The unexpanded beads are of course primarily comprised of one or morepolymers having the characteristics described above. Preferably thepolymer is polystyrene or polymers produced by polymerizing monomerswhich are derivatives of polystyrene. In general, the bead is comprisedof polymer in an amount of from about 93 to about 98 weight percent,based on total bead weight. Preferably the bead is comprised of polymerin an amount of from about 94 to about 97.5 weight percent. Still morepreferably the bead is comprised of polymer in an amount of from about95 to 97 percent, and most preferably the bead is comprised of polymerin an amount of about 96 weight percent.

If a crosslinking polyvinyl compound is present, it should not bepresent in an amount which produces an undesirably high amount ofcrosslinking. This is because the polymer will not undergo expansion ifthe degree of crosslinking is too high. The crosslinking agents whichmay be employed in the process of the present invention comprisedivinylbenzene, diethylene glycol dimethacrylate, diisopropenylbenzene,diisopropenyldiphenyl, diallylmaleate, diallylphthalate, allylacrylates,allylmethacrylates, allylfumarates, allyllitaconates, alkyd resin types,butadiene or isoprene polymers, cyclooctadiene, methylene norbornylenes,divinyl phthalates, vinyl isopropenylbenzene, divinyl biphenyl, as wellas any other di- or poly-functional compound known to be of use as acrosslinking agent in polymeric vinyl-addition compositions.

EXAMPLE 1 Method of Making Polymer

A mixture of 87 parts of water, 0.16 parts of sodium pyrophosphate, and0.27 parts of magnesiun sulfate heptahydrate was reacted with stirringat ambient temperature in a stainless steel pressure resistant vessel.To this mixture was added a mixture of 100 parts of styrene, 014 partsof benzoyl peroxide, 0.32 parts t-butylperbenzoate, 0.62 parts ofhexabromocyclododeane, and 0.21 parts of dicumyl peroxide, withstirring. The vessel was heated for at least 2 hours at a constant rateto 85° C. and then to 115° C. over 4.5 hours. Sixty-five to seventy-fiveminutes after the vessel reached 80° C., a 10% aqueous solution ofpolyvilylpyrrolidone was added to the reaction mixture. After anadditional 100-120 minutes, a solution of 0.10 parts of chain transferagent in 4.7 parts of h-pentane was added to the reaction vessel. Afterreaching 115° C., the vessel was held at constant temperature for 3hours, whereupon it was cooled to ambient temperature over 3 hours.

EXAMPLE 2

A polystyrene polymer was prepared substantially as described inExample 1. The polymer contained approximately 3.1 percent pentane(blowing agent). The resulting expandable polystyrene beads wereanalyzed according to the procedure of Example 7, and were found to havecontained polymer having a polydispersity of 1.82, a weight averagemolecular weight of 202,000, and an Mz:Mn of 2.70. The beads werescreened to 0.6-1.3 mm diameter, dried to remove surface moisture, andcoated with 0.12 weight percent of a mixture of powdered lubricants andantilumping agents commonly used in the industry as screening aids andantilumping agents. The pentane content of the beads out of thepolymerization reactor was 3.41 weight percent. However, as was typical,about 0.3 weight percent of pentane was lost during subsequentprocessing, making 3.1 weight percent the approximate pentane content atthe time of expansion.

The coated beads were expanded in a Tri Manufacturing Model 502 expanderThe inlet steam temperature was about 211° F., and the inlet steam flowrate was approximately 74 pounds per hour. The first-pass expansion ratewas about 208 pounds per hour and the outlet density of the prepuff wasabout 1.9 pounds per cubic foot. A fluidized bed drier (as commonly usedin the industry) was utilized to cool and partially stabilize theresulting prepuff. The fluidized bed dryer was equipped with a blowerwhich fluidized a portion of the beads with ambient air. The prepuff wasthen pneumatically conveyed to storage bags and aged at ambienttemperature and humidity for about 3 hours. Following aging, the prepuffwas expanded again in the same expander operated at the same conditions,the result being a prepuff having a density of about 1.10 lb/cu.ft., theexpander operating at a throughput of about 217 pounds per hour. Theresulting prepuff was again passed through the fluidized bed dryer.After airveying again to storage bags and aging for about three hours,the prepuff was transferred to a Kurtz vacuum block mold (4'×8'×34") andmolded. The molding cycle consisted of presteaming vacuum to about 0.5bar absolute pressure, followed by cross-steaming and autoclaving withsteam. The resulting block was then cooled with vacuum until the foampressure was stabilized. The pressure release time was about 30 seconds,and the resultant block had an average of 10% fusion, 1% shrinkage(defined as actual block length shrinkage 24 hours after moldingcompared to actual mold length), 1.6% collapse (defined as actualthickness shrinkage in middle of block compared to actual moldthickness), and a bulk density of 1.10 lb/cu.ft. (defined as weight ofblock divided by actual mold volume).

EXAMPLE 3

This example illustrates the expansion and molding of relatively largepolystyrene beads containing 4.4 weight percent pentane. Also shown isthe effect of too short an aging time (after the second preexpansionpass but before molding) with respect to pentane materials containingconventional amounts of pentane (approximately 6 weight percent).

An expandable polystyrene bead product containing an average of 4.40weight percent pentane and a bead diameter of 1.3-1.9 mm was expanded ina Kurtz KV1000 expander equipped with a Kurtz automatic density controlsystem which adjusted inlet steam flow to achieve desired prepuff outletdensity A fluidized-bed dryer was utilized for both expansions Thefirst-pass expansion was at a rate of 2000 lbs/hr and a density of1.22-1.25 lb/cu.ft. After about two hours age, the prepuff was expandedagain, at a rate of 3000 lbs/hr to a density of 0.88-0.90 lb/cu.ft. Theprepuff was then molded on a Kurtz vacuum block mold (26"×49.5"×196").No pre-steam vacuum was used. Steam was added at a pressure of about 0.6bar for about 6 seconds cross-steam followed by about 10 secondsautoclave. Vacuum was used to cool the block in the mold. After only onehour prepuff aging, the blocks were of poor quality, i.e. poorly fusedand deformed (poor dimensional stability). After 3-4 hours prepuffaging, the blocks were molded to 0.7 bar maximum foam pressure and wereof excellent quality with a total cycle time of 160-170 seconds. Typicalcycle times with normal pentane product after 24-36 hours aging were300-360 seconds.

EXAMPLE 4

This example illustrates how the use of a 4.4% pentane formulationcompares favorably with respect to the use of a conventionalformulation. Expansion and molding results, cycle time, fusion, anddimensional stability were acheived with the 4.4% pentane formulationover a conventional 6% pentane formulation.

An expandable polystyrene bead product containing an average of 4.40%pentane and a bead diameter of 0.6-1.3 mm was expanded in a Weiser VN400expander equipped with a fluidized-bed dryer. The first-pass rate wasabout 2600 lb/hr at an outlet density of about 1.20 lb/cu.ft. Afteraging for about four hours, the prepuff was expanded again, at a rate ofabout 4000 lb/hr and at an outlet density of about 0.79-0.88 lb/cu.ft.After about one hour of aging, the prepuff was molded in a WeiserVacuCompact block mold (196"×49"×31"). Cycle times, fusion, anddimensional stability were equal to or better than that of products ofnormal (6%) pentane content which had been aged overnight (i.e. overeight hours of aging).

EXAMPLE 5

This example illustrates, among other results, the advantageousperformance of a formulation comprising 3.6% pentane. Note the highlydesirable low aging time as well as the desirable molding cycle timewith accompanying high dimensional stability after molding.

An expandable polystyrene bead product containing an average of 3.58%pentane and a bead diameter of from 0.6-1.3 mm was expanded in a WeiserVN400 expander equipped with fluidizing bed drying. The first-passexpansion rate was about 3100 lb/hr at an outlet density of about 1.59lb/cu.ft. After aging about four hours, the prepuff was expanded again,at a rate of about 3400 lb/h and outlet density of about 0.84-0.86lb/cu.ft. After about one hour age, the prepuff was molded on a WeiserVacuCompact block mold. Pressure-release (i.e., cooling) time was only29 seconds. All blocks were well fused and dimensionally stable.

EXAMPLE 6

This example illustrates, among other advantages, how a formulationcomprising 3.63 weight percent pentane permits a highly advantageousmolding cycle time, with accompanying 50% increase in productivity inthe molding step, due to the lower molding cycle time. Good fusion anddimensional stability are also shown.

An expandable polystyrene bead product containing an average of 3.63%pentane and a bead diameter of from 0.6-1.3 mm was expanded in aDingledein & Herbert VA-K2000 expander. The first-pass expansion ratewas 3000 lb/hr at an outlet density of 1.66- 1.75 lb/cu.ft. After agingfor about 24 hours, the prepuff was expanded again, at a rate of about4000 lb/hr and an outlet density of about 0.92-0.94 lb/cu.ft. Afteraging for about two hours, the prepuff was molded in a 16' TriManufacturing block mold. Well-fused, dimensionally stable blocks wereproduced at a rate of about 15 blocks per hour, as compared to 10 blocksper hour for normal pentane-content beads.

EXAMPLE 7 Molecular Weight Distribution Curve Determination

The following equipment and procedure was utilized in order to generatethe molecular weight distribution curve for polystyrene polymers. Thisprocedure was utilized to both determine the molecular weightdistribution of the polystyrene polymer of the present invention, aswell as to analyze products which are herein compared and contrastedwith the polymer and formulation of the present invention.

Chromatography Equipment and Conditions

The apparatus consisted of a Waters 6000A pump with a U6K injector, aViscotek-supplied pulse dampener, two 30 cm PLGel 5 um Mixed Bedpolystyrene columns, a Viscotek Model 100 differential visometer (DV)and a Waters R401 differential refractometer (RI). The data acquisitionand analysis hardware consisted of an IBM PC AT equipped wit 640 kb RAM,a 30 Mb fixed disk and two 5.25" floppy disk drives; a dot matrixprinter and an HP 7475A plotter. The software used was Unical Ver. 3.11(an ASYST-based package) modified to display M_(z+1) and obtained fromViscotek.

The chromatographic conditions were as follows:

Nominal flow rate: 1.0 ml/min

Solvent: THF, high purity, non-spectro grade

sample Injection Volume 0.100 ml

RI Detector

Attenuation: 16×Polarity:+

DV Detector:

Temperature: 31.0±0.1° C.

Full Scale Output: 50 Pa

PT Sensitivity: 0.2074

Data Acquisition:

Start Time: 6 min.

Stop Time: 24 min.

Analytical Procedure

The THF to be used as the mobile phase for the GPC system was filteredthrough a 0.45 um fritted filter and then degassed under an aspiratorvacuum for approximately 45 minutes. The THF and the flask were thentransferred to the GPC system and the THF maintained under a pad ofhelium. Samples were made up to a concentration of 5 mg/ml and filteredthrough a Gelman Acrodisc CR PTEE 0.45 um filter prior to injection.

Only freshly prepared solutions were used as polymer degradation wasapparent with aged solutions. All solutions were analyzed twice. The EPS(expanded polystyrene) samples were not purified by precipitation priorto dissolution and analysis, as comparison of purified and raw EPSpolymer results indicated no significant differences. To obtain accuratesolution concentrations for the unprecipitated EPS, the initial solutionconcentrations were corrected for volatiles determined by GC andcoulombmetric analysis for pentane and moisture, respectively, orgravimetrically by baking a sample for total volatiles.

To correct for fluctuations in flow rate, each chromatogram wasnormalized to the flow rate present during calibration. This wasaccomplished by calculating the ratio of the void volumes (totalexclusion volume, earliest negative peak in chromatogram) of thecalibration chromatograms to the sample chromatogram. The calibrationvoid volume was determined to be 19.72 ml. The ratio was then enteredinto the data analysis package as the corrected flow rate.

EXAMPLES 8-15

The procedure of Example 1 was substantially followed in making anpolystyrene formulation according to the present invention. This polymerformulation is identified as Example 8 in Table I (infra). The polymerformulation of Example 9 contained a blowing agent (pentane) in anamount of about 3 5 weight percent. The polymer of Example 9 wasanalyzed according to the procedure set forth in Example 7, and fromthis analysis the number average molecular weight (Mn), the weightaverage molecular weight (Mw), and the z-average molecular weight(M_(z)) were determined. From these values the polydispersity (PD) andthe Mz:Mn were calculated.

The analytical procedure of Example 7 was also performed for severalcurrent commercial products (i e. Examples 9-14), each of whichcontained blowing agent in an amount of 5.5 weight percent to at least 7weight percent. Example 10 had blowing agent therein at a level ofapproximately 6 weight percent. From this analysis, the same molecularweight determinations were made. Table I (below) provides the results ofthe analyses for both the formulation of the present invention (Example9) as well as several commercial formulations currently available.

As can be seen from Table I, only the formulation of the presentinvention had all three identifying characteristics within the scope ofthose which are identified as pertaining to the polymer of the presentinvention. Even though each of these polymers falls within thedefinition of the polymer utilized in the formulation of the presentinvention, none of these formulations had the amount of blowing agentrequired in the formulation of the present invention.

                  TABLE I                                                         ______________________________________                                        POLYMER CHARACTERISTICS                                                                M.sub.n x10E5                                                                           M.sub.w x10E5  M.sub.z x10E5                               EXAMPLE  (g/mol)   (g/mol)   PD   (g/mol)                                                                              M.sub.z :M.sub.n                     ______________________________________                                         8       1.12      2.02      1.82 3.00   2.70                                  9       1.12      2.17      1.95 3.35   2.99                                 10       0.87      1.93      2.20 3.11   3.56                                 11       1.37      2.89      2.10 5.28   3.84                                 12       1.20      2.51      2.09 4.58   3.81                                 13       1.01      1.89      1.88 3.07   3.04                                 14       1.13      2.20      1.94 3.83   3.37                                 15       1.27      2.73      2.15 5.03   3.96                                 ______________________________________                                         [polydispersity was calculated as M.sub.w /M.sub.n, and the ratio of          M.sub.z to M.sub.w was calculated by dividing the value obtained for Mz b     the value obtained for Mn]-                                              

EXAMPLES 16-17

These two examples illustrate the difference in both (1) totalemissions, as well as (2) emissions during aging, for expandedpolystyrene which was produced according to the process described inExample 1. Example 16 illustrates the emissions from a "conventional"process which employs a blowing agent (pentane) in an amount of 6 weightpercent. Example 17 illustrates, in contrast, the emissions from aprocess employing a formulation which comprises only 3.5 weight percentpentane. For Example 16 the amount of blowing agent added during thepolymerization was approximately 6 weight percent, whereas for Example17 only about 3.5 weight percent blowing agent was added during thepolymerization. Table II (infra) provides the results of emissionsduring each of the expansion steps for each Example, for each agingperiod for each Example, and during the molding step for each Example.The bottom row of Table II provides figures for the total emissionsduring the entire process of expansion, aging and molding.

As can be seen from the figures in Table II, the polystyrene having thelow initial level of blowing agent (i.e. Example 17) exhibited a totalemissions of only from 42 to 79 percent as much as for Example 16.Furthermore, the total emissions during aging was only from about 17percent to about 48 percent for Example 17 as compared with Example 16.Accordingly, the formulation (and polymer) of the present inventionexhibit a substantial reduction in both the total emissions as well asthe emissions during aging.

                  TABLE II                                                        ______________________________________                                                      Example 16                                                                             Example 17                                             ______________________________________                                        blowing agent content                                                                         6 wt. percent                                                                            3.5 wt. percent                                    1st Pass Expansion                                                                            .76        .45                                                1st pass aging emissions                                                                      2.0        .12                                                1st pass aging time                                                                           24 hours   4 hours                                            2nd pass Expansion                                                                            N/A        .07                                                2nd pass aging emissions                                                                      N/A        .22                                                2nd pass aging time                                                                           N/A        (2 hours)                                          molding emissions                                                                             0.6-1.1    0.76%                                              Total Emissions 3.36-3.86% 1.62%                                              ______________________________________                                    

EXAMPLE 18 Method of Determining and Calculating the Degree of Branching

Branching can be determined using a viscotek differential viscometer andrelated software according to the theory of Zimm and Stockmayer. Moreextensive and descriptive discussions of this theory, as well as relatedsubject matter, can be found in Billmeyer, F. W., Jr., Textbook ofPolymer Science, 2nd Ed., 1971, Wiley-Interscience, N.Y., N.Y.,especially pages 89-90, which is herein incorporated by reference. Usingthe Mark-Houwink constants for known linear samples of polystyrene, onecan calculate the branching frequency of the number of branches per 100monomer units. More extensive and descriptive discussions of theseconstants, as well as related subject matter, can be found in Billmeyer,F. W., Jr., Textbook of Polymer Science, 2nd Ed.,1971,Wiley-Interscience, N.Y., N.Y., especially pages 86-87, which areherein incorporated by reference.

We claim:
 1. A process for making a molded, closed-cell, foamedthermoplastic expanded polymeric product while emitting only a smallamount of a volatile organic blowing agent, the process comprisingA.preexpanding unexpanded polymer beads in from 2 to 4 preexpansion steps,in an expander and at substantially atmospheric pressure wherein thepreexpansion steps are carried out so that finally preexpanded beads areproduced, the unexpanded polymer beads being comprised of:i. a blowingagent in an amount of from about 2 to 4.4 weight percent based on theweight of the beads, wherein the blowing agent is at least one memberselected from the group consisting of:pentane, cyclopentane,methylcyclopentane, neopentane, isopentane, pentane petroleum distillatefractions, propane, butane, isobutane, hexane, isomers of hexane,2-methyl pentane, 3-methyl pentane, 2,2-dimethylbutane,2,3-dimethylbutane, methylcyclohexane, cyclohexane, heptane, propylene,1-butylene, 2-butylene, isobutylene, mixtures of one or more aliphatichydrocarbons having a molecular weight of at least 42 and a boilingpoint not higher than 95° C. at 760 millimeters absolute pressure,water, carbon dioxide, ammonium carbonate, and azo compounds that aredecomposable to form a gas at a heat-plastifying temperature to whichthe polymer is brought, and ii. a polymer produced from at least onemonomer, wherein the monomer is at least one member selected from thegroup consisting of:styrene, derivatives of styrene, vinyltoluene,mono-and polyhalogenated vinyltoluenes which form linear polymers,phenyl ether, acrylonitrile, methyl methacrylate, and vinyl toluene, thepolymer being present in the beads in an amount of from about 93 weightpercent to about 98 weight percent based on the weight of the beads, thepolymer exhibiting:(a) a polydispersity of from about 1 to less than2.5, (b) a weight average molecular weight of greater than about 180,000to about 300,000, and (c) an M_(z) :M_(n) of from about 2 to about 4.5,wherein the polystyrene polymer is branched to from 0 to less than 5weight percent, and B. molding the finally-preexpanded beads in order toboth fuse and further expand the finally preexpanded beads, so that amolded foamed object having a density of from about 0.8 pounds per cubicfoot to about 2 pounds per cubic foot is formed;wherein the totalblowing agent emitted during the preexpansion steps, intermediate agingsteps, and the molding step, is only from about 1 to about 2.5 weightpercent based on the weight of the beads.
 2. A method as described inclaim 1 wherein the blowing agent is at least one member selected fromthe group consisting of pentane, cyclopentane, neopentane, isopentane,pentane petroleum distillate fractions, propane, butane, isobutane,hexane, cyclohexane, heptane, propylene, butylene, mixtures of one ormore aliphatic hydrocarbons having a molecular weight of at least 42 anda boiling point not higher than 95° C. at 760 millimeters absolutepressure, water, carbon dioxide, and ammonium carbonate.
 3. A method asdescribed in claim 1 wherein the blowing agent is at least one memberselected from the group consisting of pentane, cyclopentane, neopentane,isopentane, pentane petroleum distillate fractions, propane, butane,isobutane, hexane, cyclohexane, and heptane.
 4. A method as described inclaim 1 wherein the blowing agent is present in the unexpanded beads inan amount of from about 2.5 weight percent to about 4.4 weight percent.5. A method as described in claim 1 wherein the blowing agent is presentin an amount of from about 3 weight percent to about 4 weight percent.6. A method as described in claim 1 wherein the blowing agent is presentin an amount of about 3.5 weight percent.
 7. A method as described inclaim 1 wherein the polystyrene polymer is produced from at least onemonomer which is at least one member selected from the group consistingof styrene and derivatives of styrene.
 8. A method as described in claim1 wherein the polymer comprises from about 94 to about 97.5 weightpercent of the weight of the beads.
 9. A method as described in claim 1wherein the polymer comprises from about 95 to about 97 weight percentof the weight of the beads.
 10. A method as described in claim 1 whereinthe polymer comprises from about 96.5 weight percent of the weight ofthe beads.
 11. A method as described in claim 1 wherein from 2 to 3expansion steps are utilized to produce the expanded beads.
 12. A methodas described in claim 1 wherein 2 expansion steps are utilized toproduce the expanded beads.
 13. A method as described in claim 1 whereinthe flow rate of the beads through the expander is from about 5lb/hr/cu.ft. to about 120 lb/hr/cu.ft.
 14. A method as described inclaim 1 wherein the flow rate of the beads through the expander is from7 lb/hr/cu.ft. to about 100 lb/hr/cu.ft.
 15. A method as described inclaim 1 wherein the flow rate of the beads through the expander is fromabout 12 to about 80 lb/hr/cu.ft.
 16. A method as described by claim 1wherein the expansion steps are carried out by exposing the beads tosteam.
 17. A method as described by claim 1 wherein the steam has beenheated to a temperature of from about 200° F. to about 220° F. uponentering the expander.
 18. A process as described in claim 1 wherein thepolydispersity is from about 1.0 to less than 2.0.
 19. A process asdescribed in claim 1 wherein the polydispersity is from about 1.5 toless than 2.0.
 20. A process as described in claim 1 wherein thepolydispersity is from about 1.7 to about 1.98.
 21. A process asdescribed in claim 1 wherein the weight average molecular weight is fromgreater than about 190,000 to about 250,000.
 22. A process as describedin claim 1 wherein the weight average molecular weight is from about200,000 to about 220,000.
 23. A process as described in claim 1 whereinthe M_(z) :M_(n) is from about 2.5 to about 3.3.
 24. A process asdescribed in claim 1 wherein the M_(z) :M_(n) is from about 2.7 to about3.0.
 25. A process as described in claim 1 wherein the polystyrenepolymer is branched to from 0 to about 2 weight percent.
 26. A processas described in claim 1 wherein the polystyrene polymer is branched tofrom 0 to about 1 weight percent.
 27. A process as described in claim 1wherein the polystyrene polymer is substantially homopolymeric.
 28. Aprocess as described in claim 1 wherein the polystyrene polymer issubstantially unsubstituted.
 29. A process as described in claim 1wherein the formulation further comprises a chain transfer agent.
 30. Aprocess as described in claim 1 wherein the formulation furthercomprises a flame retardant which is an organic bromine or chlorineflame retardant compound present in an amount of from about 0.2 to about2 weight percent, based on the weight of the total formulation.
 31. Aprocess as described in claim 1 wherein the formulation furthercomprises a flame retardant consisting of a brominated hydrocarbon flameretardant being present in an amount of from about 0.5 to about 1.5weight percent, based on the weight of the total formulation.
 32. Aprocess as described in claim 1 wherein the flame retardant is at leastone member selected from the group consisting of trisdibromopropylphosphate, hexabromocyclododecane and allyl ether oftetrabromo-bis-phenol A, the flame retardant being present in an amountof from about 0.6 weight percent to 1.2 weight percent, based on thetotal weight of the formulation.
 33. A process as described in claim 1wherein the formulation further comprises a flame retardant synergistwhich is at least one member selected from the group consisting ofdicumyl peroxide and other organic peroxides which have a half-life of 1hour at temperatures of from about 110° C. to about 150° C.
 34. A methodas described in claim 1 wherein from 2 to 3 premolding expansion stepsare utilized to produce the finally-preexpanded beads.
 35. A method asdescribed in claim 1 wherein 2 premolding expansion steps are utilizedto produce the finally-preexpanded beads.
 36. A process as described inclaim 1 wherein the amount of blowing agent emitted in the expansionsteps is only from about 1.5 to about 1.9 weight percent based on thetotal weight of the beads.
 37. A process as described in claim 1 whereinthe amount of blowing agent emitted in the expansion steps is only fromabout 1.6 to about 1.8 weight percent based on the total weight of thebeads.
 38. A process for making a molded expanded polystyrene productwhile emitting only a small amount of a volatile organic blowing agent,the process comprising:A. preexpanding unexpanded beads in from 2 to 3expansion steps, the preexpansion steps being carried out in an expanderat substantially atmospheric pressure, the preexpansion steps beingcarried out so that finally-expanded beads are produced, the beads beingcomprised of:1. a blowing agent which is at least one member selectedfrom the group consisting of:pentane, cyclopentane, neopentane,isopentane, pentane petroleum distillate fractions, propane, butane,isobutane, hexane, cyclohexane, and heptane, the blowing agent beingpresent in the unexpanded beads in an amount of from about 3 weightpercent to about 4 weight percent based on the weight of the beads; and2. a polymer produced from at least one monomer, wherein the monomer isat least one member selected from the group consisting of styrene andderivatives of styrene, the polymer being present in the beads in anamount of from about 94.6 weight percent to about 97 weight percentbased on the weight of the beads, the polymer exhibiting:(i) apolydispersity of from about 1.5 to about 2.0, (ii) a weight averagemolecular weight of greater than about 190,000 to about 250,000, and(iii) an M_(z) :M_(n) of from about 2.5 to about 3 3, wherein thepolystyrene polymer is branched to from 0 to less than 2 weight percent,and B. molding the finally-preexpanded beads in order to fuse andfurther expand the finally preexpanded beads, so that a molded foamedobject having a density of from about 0.8 pounds per cubic foot to about1.1 pounds per cubic foot is formed;wherein the total blowing agentemitted during the preexpansion steps, intermediate aging steps, and themolding step is only from about 1.5 to about 1.9 weight percent based onthe weight of the beads.
 39. A process for making a molded expandedpolystyrene product emitting only a small amount of a volatile blowingagent, the process comprising the steps of:A. expanding unexpanded beadsin 2 expansion steps, wherein the expansion steps are carried out in anexpander and at substantially atmospheric pressure, and wherein theexpansion steps are carried out so that finally-expanded beads areproduced, the beads being comprised of:1. a blowing agent which is atleast one member selected from the group consisting of pentane andisomers of pentane, the blowing agent being present in the unexpandedbeads in an amount of about 3.5 weight percent, based on the weight ofthe beads; and
 2. a polymer produced from at least one monomer, whereinthe monomer is at least one member selected from the group consisting ofstyrene and derivatives of styrene, the polymer being present in thebeads in an amount of from about 96.5 weight percent based on the weightof the beads, the polymer exhibiting:(i) a polydispersity of from about1.7 to about 1.98, (ii) a weight average molecular weight of greaterthan about 200,000 to about 220,000, and (iii) an M_(z) :M_(n) of fromabout 2.7 to about 3.0, wherein the polystyrene polymer is branched tofrom 0 to less than 1 weight percent, and B. molding thefinally-preexpanded beads in order to both fuse and further expand thefinally preexpanded beads, so that a molded foamed object having adensity of from about 0.8 pounds per cubic foot to about 1.1 pounds percubic foot is formed, andwherein the total blowing agent emitted duringthe preexpansion steps, the intermediate aging steps and the moldingstep is only from about 1.6 to about 1.8 weight percent based on theweight of the beads.